The present invention relates to laser diodes and more particularly to a modulator for energizing such diodes at high frequencies and wide bandwidth.
Laser diodes are increasingly being employed for signal and data transmission, particularly in conjunction with fiber optic links. As the optical links are improved, it becomes increasingly important to develop methods of modulating the light sources which are needed to utilize such links.
Many intrinsic semiconductor lasers have large potential modulation bandwidths but are difficult to modulate due to their low impedance. Typically, the signal energizing the diode is applied through a bond wire. The parasitic inductance of the bond wire tends to dominate the small signal input impedance at microwave frequencies. The diode thus appears as a series combination of an inductance and a small resistance, e.g. 5 ohms.
Heretofore, it has been typical to utilize a resistor in series with a laser diode in order to realize an apparent input impedance of approximately 50 ohms, an impedance which is consistent with most current high frequency signalling schemes. This method is highly inefficient since most of the power is dissipated in the matching resistor. Further, the length of the bond wire imposes an upper frequency limit due to the parasitic inductance noted above.
Among the several objects of the present invention may be noted the provision of apparatus for modulating a laser diode at relatively high, e.g. microwave, frequencies; the provision of such apparatus which is relatively efficient in coupling power into the laser diode; the provision of such apparatus which avoids limitations otherwise imposed by the presence of a bond wire for coupling electrical signals to the diode itself; the provision of such apparatus which accepts a modulating signal at conventional impedance levels; the provision of such apparatus which is highly reliable and which is of relatively simple and inexpensive construction. Other objects and features will be in part apparent and in part pointed out hereinafter.